The present invention relates to a surface acoustic wave device (hereinafter called SAW filter) having corresponding comb-like electrodes.
In generally, an SAW filter is small in size, light in weigh and having high reliability. The SAW filter is used in a cellular phone and the like in a filed of a mobile communication system.
An intermediate frequency (IF) filter which is one of constituent components of the cellular phone is required to have narrow-band steep frequency characteristics with low loss, flat group delay characteristics and phase characteristics. In this SAW filter, an interdigital transducer (IDT) of a transversal filter type filter is used for obtaining the flat group delay characteristics.
Further, since narrow-band steep characteristics are required, a piezoelectric substrate in which the temperature dependence of frequency characteristic is small such as a crystal is used.
Further, it is known to set the transmitting direction of surface acoustic wave in the IDT into one direction to reduce insertion loss.
One type of unidirectional transducers is described in Jpn. Pat. Appln. KOKAI Publication No. 61-6917, for example. The unidirectional transducer described in Jpn. Pat. Appln. KOKAI Publication No. 61-6917 is of a structure so-called split electrodes or double electrodes in which a pair of comb-like electrodes are opposed on a piezoelectric substrate.
This unidirectional transducer comprises first and second opposed comb-like electrodes each including a large number of sets of electrode fingers, while each set including two electrode fingers. The first and second comb-like electrodes are arranged such that the sets of the comb-like electrodes are interdigitaly overlapped with each other. Here, when the two electrode fingers (first set) of the first comb-like electrodes are interdigitaly overlapped with the two electrode fingers (second set) of the second comb-like electrodes, a maximum distance in widthwise direction of disposition area of the four electrode fingers is .lambda. (wavelength of center frequency of surface acoustic wave). By selecting the width of the electrode finger within a range of this .lambda., a propagation direction of the surface acoustic wave is emphasized. However, in the case of the SAW filter, in addition to the selection of the propagation direction of the surface acoustic wave, it is necessary to pay attention to the fact that amplitude ripple and group delay ripple in a using frequency band must be suppressed. That is, in IDT, since triple transit echo (TTE: electric spurious multiple reflection) is generated, it is necessary to suppress the TTE.
In the above-described conventional SAW, measures for deleting the amplitude ripple and the group delay ripple in the using frequency band are insufficient.